6 the Rio Grande Project - Argentina

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The Rio Grande project - Argentina

Introduction

The Rio Grande pumped storage project is located on the Rio Grande river near thetown of Santa Rosa de Calamucita in the Province of Cordoba in Argentina. It has aninstalled capacity of 1000 MW and provides electrical storage facilities for the powergrid and, in particular, for a nuclear power plant about 50 km away from Rio Grande.

The project is owned by Agua y Energia Electrica, one of the principal Argentineanelectrical utility organisations. Preliminary feasibility studies were carried out by theowner and these were followed by detailed design studies by Studio G. Pietrangeli ofRome. The scheme was partly financed by Italy and some of the construction was

done by Condote de Agua, an Italian contractor. Golder Associates were involved inthe design and supervision of support installed to control the stability of most of themajor underground excavations.

The main underground facilities are located in massive gneiss of very good quality.The upper reservoir is impounded behind a rockfill dam and water is fed directly fromthe intakes down twin penstocks which then bifurcate to feed into the four pump-turbines. These turbines, together with valves and the control equipment, are housedin a large underground cavern with a span of 25 m and a height of 44 m.

Draft tubes from the turbines feed into twin tunnels which, with a down-stream surgeshaft, form the surge control system for this project. The twin tunnels join just

downstream of the surge tank and discharge into a single tailrace tunnel with a spanof 12 m and height of 18 m. This tailrace tunnel is about 6 km long and wasconstructed by a full-face drill-and-blast top heading, with a span of 12 m and heightof 8 m, followed by a 10 m benching operation. A view of the top heading is given inFigure 1.

Tailrace tunnel support

Because of the excellent quality of the gneiss, most of the underground excavationsdid not require support and minimal provision for support was made in the contractdocuments. Assessment of underground stability and installation of support, whererequired, was done on a design-as-you-go basis which proved to be very effective

and economical. Recent reports from site, many years after the start of constructionand commissioning of the plant, show that there have been no problems with rockfallsor underground instability.


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Figure 1: The 12 m span 8 m high top heading for the tailrace tunnel was constructedby full-face drill-and-blast and, because of the excellent quality of the massive gneiss,was largely unsupported.

Figure 2: Mechanically anchored rockbolts of the type used on the Rio Grandeproject. These bolts were tensioned to 70% of their yield load upon installation andthen, at a later stage, were re-tensioned and fully grouted.


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Figure 3: A wedge failure in the roof of the top heading of the RioGrande tailrace tunnel.

Decisions on support were made on the basis of inspection of the excavated faces bya resident team of geotechnical engineers. Where the appearance of the face indicatedthat a zone of heavily jointed rock, usually associated with faulting, was beingentered, the top heading was reduced to a 6 m span by 8 m high pilot tunnel to limitthe volume of unstable rock which could be released from the roof. This pilot tunnelwas large enough to accommodate the seven-boom jumbo, as illustrated in Figure 4,but small enough to limit the size of roof falls to manageable proportions. Boltingfrom inside the pilot heading was used to pre-support the potentially unstable wedgesand blocks in the roof.

In the case of the tailrace tunnel, which is itself a large excavation, the support

comprised mechanically anchored and cement grouted rockbolts as illustrated inFigure 2, with mesh reinforced shotcrete where required. These bolts were generallyinstalled to control the type of wedge failure illustrated in Figure 3. In the case ofparticularly large wedges, calculations of the factor of safety and supportrequirements were carried out on a programmable calculator, using an early versionof the program UNWEDGE.


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Figure 4: A 6 m wide heading driven ahead of the tunnel face to permit pre-reinforcement of potentially unstable wedges in the roof. The seven-boom jumbo isseen working in the heading.

Support for power cavern

A cross-section of the power cavern is given in Figure 5 and this figure includes thefive main excavation stages for the cavern. Careful mapping of significant structuralfeatures in the roof and walls of the central access drive at the top of the cavernprovided information for estimating potentially unstable blocks and wedges whichcould form in the roof of the cavern. Figure 6 illustrates a number of such wedges inone section of the cavern roof. At each stage of the cavern excavation, long rockbolts(up to 10 m length) were installed to stabilise wedges or blocks which had beendetermined as being potentially unstable.

Because gneiss has usually undergone some tectonic deformation during its

geological history, projection of structural features from visible exposures tends to bean imprecise process. Consequently, the potentially unstable blocks and wedges hadto be reassessed after each excavation step revealed new information. The structuralplan illustrated in Figure 6 had to be modified many times during excavation and thatshown is the final plan prepared after the full cavern roof had been exposed.


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A general view of the cavern excavation is given in Figure 7. This photograph wastaken when the bulk of the cavern had been completed and only a few benches in thebottom of the cavern remained to be excavated. The enlarged top of the cavern is toaccommodate the overhanging crane that is supported on columns from the cavernfloor. An alternative design for this cavern would have been to support the crane on

concrete beams anchored to the walls as is commonly done in good quality rock.

Figure 5: Cavern profile and excavation stages.


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Figure 6: A plan of the traces of geological features mapped in part of the cavernroof. The shaded areas represent potentially unstable wedges requiring reinforcement.


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Figure 7: A view of the 25 m span Rio Grande power cavern during excavation of thelower benches.

Discussion of support design and costs

Apart from rockbolts installed to control isolated structurally controlled blocks andwedges in the roof and sidewalls and some areas of closely jointed rock which wereshotcreted, the cavern was unsupported. While this was successful for this particularproject, it is not the approach which should generally be used for a critical excavationsuch as an underground powerhouse.


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The damage resulting from even a small rockfall in such a cavern is out of allproportion to the savings achieved by eliminating pattern rockbolting and fullshotcrete lining. Hence, in addition to the rockbolts installed to control structuralinstability, as described earlier, I would recommend a normal pattern of 25 mmdiameter, 5 m long bolts (20% of the excavation span) on a 2.5 m grid. In addition, I

would recommend the placement of 50 mm of fibre-reinforced micro-silica shotcreteover the entire roof and upper sidewalls of the cavern. Based on current northAmerican costs, this additional support, involving approximately 600 rockbolts andabout 300 m3 of shotcrete, would have cost approximately US $200,000. In terms ofthe overall project cost and the increased long-term security in the cavern, this wouldnormally be regarded as a good investment.

In contrast, consider the 6 km long tailrace tunnel in which the consequences of asmall rockfall are minimal. Assume that a pattern of 4 m long bolts on a 2 m grid (say10 bolts per section) and a 50 mm shotcrete thickness had been specified for the roofand upper sidewalls of the tailrace tunnel. This would involve 30,000 bolts and 5,400

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of shotcrete at a total cost approaching US $5 million. This example illustrates theneed to give careful consideration to the function and risks associated with eachunderground excavation before deciding upon the support system to be used.

Analysis using UNWEDGE program

UNWEDGE1 is a user-friendly micro-computer program which can be used toanalyse the geometry and the stability of wedges defined by intersecting structuraldiscontinuities in the rock mass surrounding an underground excavation. The analysisis based upon the assumption that the wedges, defined by three intersectingdiscontinuities, are subjected to gravitational loading only. In other words, the stressfield in the rock mass surrounding the excavation is not taken into account. While this

assumption leads to some inaccuracy in the analysis, it generally leads to a lowerfactor of safety than that which would occur if the in situ stresses were taken intoaccount.

The application of the program UNWEDGE to the analysis of a potentially unstablewedge in the Rio Grande cavern is illustrated in the following discussion.

Input Data

The dips and dip directions of a number of planes can be entered directly into thetable which appears when the Input data option is chosen or this information can beentered in the form of a DIPS file. Once the data has been read into the program, the

great circles representing the discontinuities are displayed on the screen as illustratedin Figure 8 and the user is prompted to select the three joint planes to be included inthe analysis. Alternatively, the program can be instructed to compute the three mostcritical planes those giving the largest wedges with the lowest factors of safety.Once the information on these planes has been entered, the unit weight of the rock

1 Available from www.rocscience.com


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and the shear strengths of the joints are entered. Finally, the water pressure acting onthe joint surface is entered. In most cases, the default water pressure of 0 will bechosen but the user may check the sensitivity of the wedge to pore water pressure byentering appropriate values.

In the case of the rock mass surrounding the Rio Grande Cavern, the dips and dipdirections of the following three sets of joints are included in Figure 8:

1 88/225 shear joint set2 85/264 shear joint set3 50/345 tension joint setCavern axis: trend 158, plunge 0

Figure 8: Great circles representing four joint sets which occur in the rock masssurrounding the Rio Grande cavern - imported as a DIPS file.

Input of excavation cross-section

In setting up this analysis, the co-ordinates shown in Figure 9 were used to define thecavern profile. These co-ordinates must be entered sequentially and must form aclosed figure. The profile is formed from straight line and arc segments and asufficient number of co-ordinates should be entered to ensure that a smooth profile isgenerated.


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Determination of wedge geometry

Depending upon the shape of the cross-section, a maximum of six wedges can beformed with three intersecting joint planes. Selecting the 3D wedge view optiongives a number of views showing the shape and size of these wedges. The two

wedges formed on the cavern end walls can be viewed by activating the End wedgesoption.

Figure 10 shows the wedges formed in the case of the Rio Grande power cavern forthe three joint planes defined in Figure 8. The weight of each of these wedges, thefailure mode and the calculated factor of safety are shown in the figure. Obviously,the most dangerous wedge in this situation is the wedge formed in the roof while thewedge formed in the floor is stable and need not be considered further in this analysis.

Figure 9: Co-ordinates used to define the profile of the cavern.


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Figure 10: Perspective view of the wedges formed in the rock mass surrounding theRio Grande power cavern.

Sizing or wedges

The program UNWEDGE automatically determined the largest wedge that can occurin the rock mass adjacent to the excavation profile. In the case of the roof wedge,shown in Figure 10, the wedge extends over the full 25 m span of the cavern andweighs 11,610 tonnes. While, in exceptional circumstances, such wedges may occur,the limited extent of joints in many rock masses will restrict the size of the wedges tomuch smaller dimensions than those determined by UNWEDGE for the largeexcavations.

As illustrated in Figure 6, the trace length of joint number 3 (50/345) in the upperroof wedge is approximately 6 m. When the Scale wedges is chosen, the user can

define the size of the wedge in terms of the area of the face on the excavation surface,the volume of the wedge, the height of the apex of the wedge, the length of one of thejoint traces or the persistence of one of the joints. In this case a trace length of 6 m isentered for joint number 3, defined by 50/345, and the resulting wedge is illustrated inFigure 11. This wedge weighs 220 tonnes and will require about seven 50 tonnecapacity fully grouted cables to give a factor of safety of about 1.5 which isconsidered appropriate for a cavern of this type.


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Figure 11: Perspective view of roof wedge in the Rig Grande cavern roof. The size ofthis wedge has been defined by setting the trace length of the 50/345 joint to 6 m.Eight 10 m long 50 tonne capacity grouted anchors give a factor of safety of 1.6 .

UNWEDGE allows the user to add a layer of shotcrete and calculates the factor ofsafety increase as a result of such an addition. Since the shotcrete can only be addedonce the surface of the wedge is fully exposed it is not taken into account incalculating the support required to stabilise the wedge. The increase in safety factorwhich occurs after the shotcrete has set can be regarded as a long term bonus and itdoes allow the user to choose a slightly lower factor of safety for the immediatesupport of the wedge.

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